Known laser beam levelling devices serve to determine a horizontal or an inclined plane and are used for building purposes. The laser beam issuing from the device is received at a relatively great distance by a receiver. The inclination of the imaginary plane between the laser beam device and the receiver may therefore be measured. Laser beam levelling devices are used, for example, in ground excavation and the grading of pits, in the monitoring of concrete foundations and concrete ceilings, orientation of formwork, in the levelling of tracks for building cranes and much more.
Canal building laser devices are a subgenus of laser beam levelling devices. They are used in the construction of pipelines, for example of drainage pipes, in order to lay the individual lengths of pipe in the desired inclination and direction. The lengths of pipe are lined up until they form a pipeline having an inclination of, for example, 1.2%. Shafts are installed at intervals; there is no change of direction between the shafts. The inclination of the individual portions of pipeline between the shafts has to differ in each case in practice.
During the construction of a pipeline, the canal building laser device is erected at the beginning of a line, i.e. normally in the region of a shaft, such that the laser beam points in the desired direction and has the desired inclination. The laser beam therefore indicates both the direction and the inclination in which the pipeline is to be constructed. The receiver is inserted at the opposite end of the length of pipe to be positioned. The receiver usually has a transparent disc with markings in the form of a reticule. The laser beam issuing from the canal building laser device strikes the disc of the receiver as a spot of light. As soon as this spot of light is located in the centre of the reticule, the length of tube is correctly positioned and may be fastened in this position.
The formerly known canal building laser devices contain helium neon laser tubes producing a laser beam which is visible up to a distance of 100 m depending on the optical system concentrating the laser beam.
So-called laser diodes have also recently become available. They have the advantage that they consume far less power than conventional helium neon laser tubes. Former canal building laser devices are assigned an external power source owing to the high energy consumption of the laser tubes. Furthermore, laser diodes are much smaller in their dimensions, so the overall size of the laser devices could be smaller, making them suitable for the construction of smaller diameter pipelines. The former canal building laser devices are suitable for pipelines having diameters of at least 150 mm. A drawback is that the laser beam of the formerly available laser diodes is visible only up to 40 or at most 50 m.
A further drawback is that the light spot formed by the laser beam on the transparent disc of the receiver can only be seen optimally on the side to be turned away from the laser beam--at the location of the device operator--if the angle of view is approximately in the axis of the arriving laser beam. The steeper this angle of view, the more difficult it is to see the light spot. If the distance between the canal building device and the receiver is too great, the light spot is no longer visible even if there is only a slight deviation between the angle of view and the laser beam axis. As the receiver is to be arranged in the pipe located on the bottom of a pit, the practical range of action of the canal building laser device is reduced considerably if the device operator does not lower himself onto the possibly muddy bottom of the pit in order to observe the light spot.